1. Conclusion QDs embedded in micropillars are fabricated by MOCVD and FIB post milling processes with the final quality factor about 4500. Coupling of single QD to the fundamental cavity mode is tuned by varying temperature and magnetic field, and the enhancement of single dot’s emission is observed at resonance. Control of the exciton spin states is realized by selective coupling of the cavity and QD excitons at specific spin state tuned by magnetic field. Growth and Fabrication of GaAs/AlAs MCs We report the design, fabrication and magneto-optical investigation of the single quantum dot (QD)–micropillar cavity system. An efficient magnetic field induced tuning of the QD-cavity coupling, based on the effects of diamagnetic shift and Zeeman splitting of the confined excitons, is demonstrated. A profound enhancement of the QD luminescence is observed at resonant coupling. By means of large Zeeman splitting, the light emission from excitons at one spin state is selectively enhanced with respect to the other by a factor of 3, showing the effective control of the spontaneous emission via magnetic field and Purcell effect. Our results provide an alterative for realizing polarized single photon devices in the cavity-quantum electradynamics regime. Selective Enhancement of Photon Emission in a Quantum Dot Coupling with Micropillar Cavity system Jian Lu 1, Qijun Ren 1, H. Tan 2, Shan Wu 3,Yongyuan Zhu 3, C.Jagadish 2, Xuechu Shen 1 and Zhanghai Chen 1* 1 State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai, 200433, China 2 Department of Electronic Materials Engineering, Australian, National University, Canberra, ACT 0200, Australia 3 National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, ChinaMotivation The two-level system(Fig.1) couple to a cavity mode can be described by the following Hamiltonian: Defining the oscillator strength : The coupling constant for the exciton–photon interaction: The new two energy eigenvalues: Fig. 1 QD–microcavity system with high Q-factor and small-modal- volume cavities for the realization of Purcell effect and strong coupling between single quantum emitter and optical modes by tuning the Temperature, Electric field, Molecule deposition… What about magnetic field? Coupling between excitont spin states and cavity, Generation of polarized single photon emission Fig.2 sample structure Fig.3 etching by FIB Sample structure (Fig.2,3) :  The active layer is made up of InAs self- assembled QDs was grown by MOCVD  The planar cavity consists of a 940nm GaAs cavity surrounded by a lower mirror consisting of a 30 pair alternating GaAs/AlAs ¼ wave pairs DBR and an upper DBR mirror of 23 ¼ wave pairs  The fabrication of micropillars is achieved by employing a FIB without gas- assisted etching and using three kind of currents to smooth the wallsides 23 stacks of top DBRs 27 stacks of bottom DBRs GaAs λ cavity λ = 936 nm Active layer of InGaAs QDs were embedded in the wavelength cavity GaAs substrate Fig4. DBR stopband and cavity Q factor Experiment and Discussion The relative intensities of the σ+ and σ- emissions shows that the exciton with σ+ spin state is selectively enhanced by the cavity. The intensity ratio is as large as 26 at σ+ -mode resonance. It provides an effective way of controlling the emission of excitons at different spin states. Excitation and detection of μPL of single pillar in normal- incidence geometry:  Excitated laser wavelength 633nm(1.96ev)  Laser spot diameter about 2~3μm  Excitation Power lower than 10 μw  Temperature change from 5K to 55K  Magnetic field from 0T to 5T The Cavity QD in Magnetic fields Fig.5 The PL mapping spectra of quantum dot and cavity mode by verying magnetic field from 0T to 5T. We modeled the system by four-level rate equations including the biexciton state. Fig.6 illustrates the level diagram of a single QD in external magnetic field. In our system, the QD is non-resonant pumped Fig.7 The ratio of and emission intensities tuned by magnetic field, demonstrates the elective enhancement of the exciton spin state emission Fig.6 four-level diagram of a single QD under magnetic field